This invention relates to switching converters, and more particularly to an improved technique for reducing power loss in the converter during turnoff of the switching transistor thereof.
A prior art switching converter comprises an inductor, load resistor, and emitter-collector path of a switching transistor electrically connected in series across a source of DC voltage, a commutating diode being connected across the series combination of the inductor and load resistor. When the switching transistor conducts in saturation, the collector current is high and the collector-emitter voltage approximately 0 volts. When the transistor starts to turn off, the emitter-collector voltage rises abruptly to its full value, whereas turnoff of collector current is delayed by the presence of minority carriers in the base region. (See curves 101 and 102 in FIG. 2) This rise in collector voltage while collector current is still flowing can cause excessive power dissipation in and result in overheating of the switching transistor. In order to decrease such power loss, the regulator in U.S. Pat. No. 3,736,495, issued May 29, 1973, "Switching Regulator With High Frequency Turnoff Loss Reduction Circuit" by Calkin, et al, connects capacitive energy storage means across the commutating diode for slowing down the rise in emitter-collector voltage of the switching transistor. Although this technique improves efficiency, it also slows down the switching operation of the regulator. A different approach is described in U.S. Pat. No. 3,697,852, Oct. 10, 1972, "Transistor Switching Regulator" by Clarence G. Gerbitz. The Gerbitz regulator requires high voltage switching transistors having emitter-collector paths electrically connected to ground through a control circuit comprising high-speed low-voltage transistors which are turned on and off. The bases of these switching transistors are connected through a shunt connected resistor and junction diode to a positive bias voltage. When transistors in the control circuit are cut off, the principal conduction path in the switching transistors is interrupted. Since collector current is still flowing there, however, their base voltages rise sufficiently to turn on the junction diode to provide reverse base drive for switching the operating point of the switching transistors more rapidly through the region of high dissipation (column 3, line 60). This technique requires a bias voltage, special control circuitry, high voltage switching transistors, and as a relatively high power loss since two transistors are conducting in series during the on-cycle.
An object of this invention is the provision of improved circuitry in a switching converter for rapidly changing the operating state of a switching transistor thereof for reducing power loss in the transistor during turnoff thereof.